1. Field of the Invention
The present invention relates generally to a method of shortening a rising time of each injection-molding operation in association with the controlling an injection-molding speed. More particularly, the present invention relates to a method of the foregoing type which assures that a rising time at the beginning of an injection molding operation to be performed by actuating a piston in an injection cylinder can be shortened, and moreover, an intensity of back pressure can be reduced to a zero level.
2. Background Art
To facilitate understanding of the present invention, typical injection-molding hydraulic circuits each employed for practicing a conventional method of the foregoing type will be described below with reference to FIG. 4 and FIG. 5.
With respect to the injection-molding hydraulic circuit shown in FIG. 4, a direction shifting valve V1 for supplying pressurized hydraulic oil to a hydraulic chamber 3 on the rearward displacement side defined by a piston 2 and discharging it therefrom, and moreover, discharging hydraulic oil from a chamber 4 on the forward displacement side and a direction shifting valve V2 for supplying pressurized hydraulic oil to the chamber 4 and a hydraulic motor M are disposed in a hydraulic circuit for an injection cylinder 1 and a hydraulic pump PV adapted to discharge pressurized hydraulic oil by a variable quantity.
On the other hand, with respect to the injection-molding hydraulic circuit shown in FIG. 5, a hydraulic pump PF adapted to discharge pressurized hydraulic oil by a constant quantity is substituted for the variable quantity type hydraulic pump PV, and a quantity of pressurized hydraulic oil to be discharged from the hydraulic pump PF is properly controlled by a proportional valve V3 disposed at the intermediate position of a hydraulic path extending between the hydraulic pump PF and a direction shifting valve V2 as well as a relief valve V4 disposed between the foregoing hydraulic path and a return circuit extending from a hydraulic motor M.
Irrespective of the structure employed for each of the conventional injection-molding hydraulic circuits shown in FIG. 4 and FIG. 5, when an injection-molding operation is to be started, the direction shifting valve V1 is displaced to the b side from the shown state, and at the same time, the direction shifting valve V2 is displaced to the a side from the shown state, whereby a hydraulic path on the rearward displacement side of the direction shifting valve V1 is hydraulically connected to an oil tank and a hydraulic path on the forward displacement side is held in the blocked state. In addition, a hydraulic path on the pump side of the direction shifting valve V2 and a hydraulic path on the forward displacement side are communicated with each other, whereby pressurized hydraulic oil is exerted on the hydraulic chamber 4 on the forward displacement side, causing the piston 2 to be displaced in the forward direction while discharging hydraulic oil from the hydraulic chamber 3 on the rearward displacement side. Thus, the injection-molding operation can be started.
In the case of the injection-molding hydraulic circuit shown in FIG. 4, a flow rate and a hydraulic pressure of hydraulic oil discharged from the hydraulic pump PV start to simultaneously rise after both the direction shifting valves V1 and V2 each having four ports are shifted to the opposite side, causing the piston 2 to be displaced in the forward direction. However, since the beginning of the forward displacement of the piston 2 is largely affected by the rising state of the hydraulic pump PV, it is practically difficult to quickly displace the piston 2 in the forward direction.
On the other hand, in the case of the injection-molding hydraulic circuit shown in FIG. 5, after the hydraulic pump PF is driven, both the direction shifting valve V1 and V2 are simultaneously actuated together with the proportional valve V3. However, the piston 2 is liable of being displaced in the forward direction with delay not only due to slight delay of the shifting of the proportional valve V3 but also due to the inertia force given by the hydraulic oil present behind the proportional valve V3.